1. Field of the Invention
The present invention relates to floating vessels used for offshore drilling and production of petroleum.
2. Description of the Related Art
Petroleum production often requires the placement of rig in an offshore location. In shallower waters, the rigs and production facilities can be placed on freestanding offshore platforms. As the water becomes deeper, however, these become impractical, and it is necessary to have a floating platform, or support vessel, upon which the rigs and production facilities can be placed.
One type of deepwater support vessel is a tension leg platform (TLP). The TLP is a buoyant platform that is secured to the seabed using generally vertically-oriented rigid tethers or rods that restrain the platform against vertical and horizontal motion relative to the well in the seabed below. These platforms have a very short period in response to wave action.
An alternative to the TLP is the deep draft caisson vessel (DDCV). The DDCV is a free floating vessel that is moored to the seabed using flexible tethers so that vertical and horizontal motion of the vessel is restrained, although not eliminated . Examples of DDCVs are found in U.S. Pat. No. 4,702,321.
Methods for restraining the DDCVs attempt to slow, rather than eliminate, the natural response period of the vessel to wave effects. Current DDCV arrangements xe2x80x9cdecouplexe2x80x9d the vessel from the individual wells being supported so that the wells are not subject to the same induced motions as the vessel. Decoupling is typically accomplished by using buoyant means to make the wells separately freestanding and using flexible hoses to interconnect the vertical risers from the well to the production facilities.
A common variety of DDCV is the type shown in U.S. Pat. No. 4,702,321, which utilizes a long cylindrical structure and is commonly known as a spar. The long cylindrical shape of the spar provides a very stable structure when the vessel is in its installed position, exhibiting very slow pitch, surge and heave motions. Heave motion, however, is not totally eliminated, allowing the structure to bob up and down vertically in the sea. Recently, attempts have been made to add a number of horizontally extending plates along the length of the spar in order to help the spar be more resistant to heave.
Regardless of the presence of the plates, the spar must be assembled and transported in a horizontal position and then installed by being upended at or near the final site using a large crane that must also be transported to the installation site. As these caisson structures are often around 650 ft. in length, transport and upending of the structure are risky. Further, it is only after a successful upending of the structure has occurred, and the lower portion of the structure has been successfully moored, that components of the rig can be placed atop the spar.
In this invention, a platform is provided that has a variable ballast. A flotation device is coupled to the platform to increase the buoyancy of the platform. The flotation device causes the platform to float in a towing position with the platform and the flotation device partially submerged. The flotation device is fixed to the platform while in the towing position, and the platform is towed upright. When at the site, the flotation device is moved to a deploying position. In the deploying position, the flotation device remains in close proximity with a portion of the platform, but is not fixed to it vertically. As ballast is increased in the platform, the platform moves downward relative to the flotation device. The flotation device remains floating closely spaced to a portion of the platform. If the platform heels while lowering, it will contact the flotation device, which provides lateral stability against heeling. Once the platform has been submerged sufficiently so that it is stable, the flotation device is released from the platform.
In the preferred embodiment, the platform has an upper elongated tower section and a lower base section. The base section has a greater cross-sectional dimension than the tower section. The flotation device is preferably annular and fits on top of the base section, surrounding a lower portion of the tower section. Preferably the flotation device is formed in circumferentially extending segments. The segments are separable from each other. The flotation device is disengaged from the platform by uncoupling the segments from each other and pulling them laterally outward from the platform.
In the first embodiment, the upper deck structure of the platform is mounted to the platform before the platform is towed to the desired location. In the second embodiment, the upper deck structure is installed at the location. This is handled by mounting the upper deck structure on a buoyant member and towing the buoyant member to the location. The buoyant member has two spaced-apart arms, resulting in a slot. The arms are spaced apart from each other sufficiently to allow the arms of the buoyant member to float on opposite sides of the platform after the platform has been fully deployed and the flotation device removed. The arms support the upper deck structure at a distance above the upper end of the platform. Once in place over the platform, the platform buoyancy is increased, allowing the platform to rise up into contact with the upper deck structure. The deck structure is then secured to the platform, and the buoyant member is then moved laterally away from the structure.